Steve_H is correct. The 'weight' of an oil is derived at the time it takes for a set amount of oil, at a SPECIFIC temp to pass through a measured orifice.

The time is then cross referenced on a chart. The chart has SOME overlap.. and the times are not really that far apart.

Read this and you will be an oil guru:

http://www.synlube.com/viscosit.htm

If you look at this chart HERE

You will see that for SAE 0W, 5W, 10W, 15W and 20 (non-W) the difference is all within 100 points.. out of a scale of a possible 10000 (or 1500 for common grades).